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SELECT key_number, dateadded,
  IF (action=@last_action AND key_number=@last_key,"ERROR","OK"), 
...
    FROM keysafe
    ORDER BY key_number, dateadded
  ) as x

$dbh->do('SET @last_action := "UNKNOWN",@last_key="UNKNOWN"');

my $sql = 'SELECT key_number,action,dateadded
           FROM keysafe
...
  $last_key    = $key;
  $last_action = $action;
}